Glass sheet and glass sheet photoelectric converter device

ABSTRACT

The present invention provides a glass sheet formed on molten tin, that exhibits a high transmittance that is originally inherent to its composition. In this glass sheet, the difference between a visible light reflectance of a surface formed in contact with the molten tin and a visible light reflectance of a surface on the opposite side thereof is not greater than 0.25%. And when the glass sheet&#39;s thickness is 4 mm, a light transmittance at 540 nm wavelength is at least 91.5%, and a light transmittance at 450 nm wavelength is at least 91.0%, and after irradiating UV light for 6 hours as specified in the radiation-proofing test according to JIS R3212, a light transmittance at 540 nm wavelength is at least 91.0% and a light transmittance at 450 nm wavelength is at least 90.5%.

TECHNICAL FIELD

The present invention relates to glass sheets formed on molten tin, andmore particularly to glass sheets for photoelectric conversion devicesthat are used in photoelectric conversion devices, such as solar cells.

BACKGROUND ART

In photoelectric conversion devices, glass sheets with hightransmittance and superior durability are used widely as the basematerial for the side on which light is incident. The glass sheets areused as cover glass for protecting the photoelectric conversion elementsand as the substrate on which the photoelectric conversion elements areformed. In the former type of glass sheets, photoelectric conversionelements made of crystalline silicon or the like are layered on theglass sheet with a resin film made of EVA (ethylene-vinyl acetatecopolymer) disposed in between, for example. In the latter type of glasssheets, photoelectric conversion elements made of thin-film silicon orthe like are formed on the surface of the glass sheet with a transparentconductive film disposed in between (see for example JP H11-298030A).

A high transmittance is desired for these glass sheets. In order toincrease the transmittance, coloring due to iron components in soda-limeglass may be suppressed by using a raw material with high purity. Forexample, JP H4-228450A discloses soda-lime glass with low ironcomponents. This glass is a transparent glass with colored edge thatcontains, in mass %, less than 0.2% total iron oxide in terms of Fe₂O₃as colorant, and wherein the ratio of ferrous oxide (FeO) to total ironoxide is at least 0.4. At a thickness of 5.66 mm, this glass has a lighttransmittance (illumination C) of at least 87%.

In order to obtain soda-lime glass having a light tint and a hightransmittance while including the ordinary amount of iron oxide, it hasbeen proposed to add an oxidizer such as cerium oxide, and to thus lowerthe content of FeO, which is the principal component responsible forcoloring and decrease in transmittance. For example, JP H5-221683Adiscloses that the Fe²⁺/Fe³⁺ ratio in the glass can be lowered from theordinary level (about 38%) to 3 to 10% by including 0.1 to 0.5 wt % ofCeO₂ as oxidizer in ordinary transparent soda-lime glass containing 0.06to 0.12 wt % of iron impurities in terms of Fe₂O₃. Thus, a hightransmittance is attained in a wavelength region of around 600 nm andhigher.

Thus, conventionally it has been attempted to improve the glass sheetsfor photoelectric conversion devices by restricting the total amount ofiron oxide or by controlling the oxidation/reduction state (i.e. theFe²⁺/Fe³⁺ ratio) of the iron oxide. These glass sheets ordinarily aremanufactured by the so-called float process, in which the molten glassare poured onto molten tin kept in a tin float bath, and formed into aglass sheet on that molten tin. However, according to an analysisperformed by the inventors, the transmittance of soda-lime glassmanufactured by the float process remains at a value that is lower thanwhat theoretically could be accomplished with the glass composition.

Other processes for manufacturing glass sheets besides the float processare known, such as the roll-out process, the down-draw process or glassshaping on a thin layer of water vapor (see for example JP H9-295819A).However, considering the required size of the glass sheet and themanufacturing costs, the float process is even today still the mostadvantageous manufacturing process not only for main applications inbuilding and vehicle glass sheets etc., but also in photoelectricconversion devices.

DISCLOSURE OF THE INVENTION

It is thus an object of the present invention to provide a glass sheetthat can exhibit the high transmittance that is originally inherent inits composition.

In glass sheets formed by the float process, the surface that has beenformed in contact with the molten tin includes trace components thathave diffused from the molten tin. Due to these trace components, therefractive index at the surface of the glass sheet that has been formedin contact with the molten tin (bottom surface) becomes slightly higherthan the refractive index inherent in the glass composition. Therefore,the light reflectance at the bottom surface becomes relatively higher(for example about 0.5%) than the light reflectance at the surface onthe opposite side that has not been formed in contact with the moltentin (top surface).

Furthermore, the trace components at the bottom surface contribute tochanges in the light transmittance of the glass sheet over time, inparticular contribute to a decrease of the light transmittance of theglass sheet when it is exposed to UV light.

In one aspect of the present invention, a glass sheet is presented inwhich the trace components at the bottom surface have been controlled.In this glass sheet, which is formed on molten tin, the differencebetween a visible light reflectance of the bottom surface and a visiblelight reflectance of the top surface is not greater than 0.25%,preferably not greater than 0.15%, and when the glass sheet's thicknessis 4 mm, a light transmittance at 540 nm wavelength is at least 91.5%,and a light transmittance of at 450 nm wavelength is at least 91.0%, andafter irradiating UV light for 6 hours as specified in theradiation-proofing test according to Japanese Industrial Standard (JIS)R3212, a light transmittance at 540 nm wavelength is at least 91.0% anda light transmittance at 450 nm wavelength is at least 90.5%.

Throughout this specification, “reflectance” of a surface of the glasssheet is defined as the value not including the reflection from thesurface opposite the surface of interest (that is, the surface on therear side).

It should be noted that in the glass sheet according to the presentinvention, even though changes in the transmittance over time aresuppressed more than in the related art, they are not completelyeliminated. Also in glass sheets to which the present invention has beenapplied, after long-term usage as a component in a photoelectricconversion device, the transmittance may decrease slightly due theinfluence of UV light. However, be it the UV light included in sunlightor the UV light from a UV light lamp as specified by the above-notedJIS, the influence of UV light irradiation on the transmittance can beeliminated by baking. To eliminate this influence, baking in air at 500°C. for 30 min is sufficient.

The present invention encompasses glass sheets in which the lighttransmittance after baking under these conditions has at least theabove-noted values (at least 91.5% at 540 nm wavelength, and at least91.0% at 450 nm wavelength), and the light transmittance aftersubsequently carrying out the above-noted UV light irradiation test hasat least the above-noted values (at least 91.5% at 540 nm wavelength,and at least 90.5% at 450 nm wavelength),

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing an example of a glass sheet fora photoelectric conversion device in accordance with the presentinvention.

FIG. 2 is a diagram showing the configuration of an apparatus that canbe applied to the fabrication of glass sheets in accordance with thepresent invention.

EMBODIMENTS OF THE INVENTION

In glass sheets manufactured by the float process (i.e., in floatglass), trace components originating from the molten tin make itpossible to distinguish the bottom surface from the top surface of theglass. The present invention suppresses an increase in trace componentsthat contribute to a rise of the refractive index of the glass, and inparticular an increase of the concentration of tin oxide and iron oxidenear the bottom surface. However, the diffusion itself of tracecomponents into the bottom surface is not entirely prevented, and thusit is still possible to distinguish bottom surface and top surface, asin the prior art.

As is widely known, the precise measurement of trace components in smallregions is not easy, and the measured values may not match completely,due to differences in the resolution of the analytical procedures forexample. Throughout this specification, trace components (tin oxide,iron oxide) near the bottom surface are specified on the basis of thevalues measured by a wavelength dispersive X-ray detector (WDX) mountedto an electron probe micro-analyzer (EPMA), which is one of the ordinarymethods for the quantitative analysis of surfaces.

In the glass sheet of the present invention, it is preferable that themaximum value of the tin oxide concentration in terms of SnO₂ (that is,converting the Sn that is ordinarily present in bivalent and tetravalentform into tetravalent) up to a depth of 10 μm from the bottom surface isnot more than 1 mass %. Moreover, it is preferable that the maximumvalue of the iron oxide concentration in terms of Fe₂O₃ (that is,converting the Fe that is ordinarily present in bivalent and trivalentform into trivalent) from the bottom surface is not more than 0.2 mass%.

Controlling the concentration of these trace components, the visiblelight reflectance at the bottom surface can be reduced to a value notgreater than 4.5%. Restricting the concentration of at least the tinoxide to the level indicated above, the visible light reflectance at thebottom surface can be reduced to a value not greater than 4.25%.Moreover, controlling both the tin oxide concentration and the ironoxide concentration, it becomes possible to reduce the visible lightreflectance at the bottom surface to a value not greater than 4.15%.Thus, a glass sheet can be obtained in which the visible lightreflectance at the bottom surface and the top surface is not greaterthan 4.5%, preferably not greater than 4.25% and even more preferablynot greater than 4.15%.

By restricting the trace components diffused from the bottom surface,the transmittance of the glass sheet can be kept high, while at the sametime restricting also temporal changes in the transmittance. Inparticular reductions of the transmittance due to UV light, small asthey may be, have a large effect on the photoelectric conversionefficiency of photoelectric conversion devices. Applying the presentinvention, in a glass sheet of 4 mm thickness, the changes in lighttransmittance at 540 nm wavelength before and after irradiating UVlight, as specified in the radiation-proofing test according to JISR3212, can be restricted to not more than 5%, or even not more than 4%.Also the changes in light transmittance at 450 nm wavelength can berestricted to not more than 5%, or even not more than 4%.

The amount of tin oxide included near the bottom surface is affected bythe purity of the molten tin. To decrease the tin oxide concentrationnear the bottom surface, the reducibility of the atmosphere in the tinfloat bath should be kept high, at such a level that oxidizing gas doesnot even partially oxidize the molten tin when such oxidizing gas isleaked into the tin float bath. Ordinarily, nitrogen gas and hydrogengas are supplied to the tin float bath, but trace amounts of tin oxidemay be generated in the molten tin due to the leak of trace amounts ofoxidizing gas, such as oxygen. Under ordinary operating conditions, thistin oxide can be tolerated if its amount is so small that it does notnegatively affect the forming of the glass sheets. However, tosatisfactorily suppress the diffusion of tin oxide into the bottomsurface of the glass sheet, it is necessary to reliably suppress thegeneration of tin oxide in the molten tin. A reliable suppression of theoxidation of the molten tin can be achieved by raising the hydrogenconcentration and raising the gas pressure in the bath.

To reduce the iron oxide concentration near the bottom surface, the ironconcentration in the molten tin should be reduced. It is preferable thatthe iron concentration in the molten tin is not greater than 100 ppm,but it does not need to be too low, and for example at least 55 ppm andless than 100 ppm is appropriate.

With the present invention, increases in reflectance due tocompositional fluctuations near the surface can be suppressed, so thatthe inherent transmittance of the glass composition can be more easilyattained. In order to attain a high transmittance, it is preferable thatthe amount of iron oxide in the glass composition is low, morespecifically, that the amount of iron oxide total in terms of Fe₂O₃ isnot greater than 0.04 mass %, and in particular not greater than 0.02mass %. In this case, it is preferable that the glass composition issubstantially free from cerium oxide. Throughout this specification,“substantially free from” means that an admixture of trace amounts istolerable and refers to ranges of not greater than 0.001 mass %, forexample.

Both bivalent iron oxide (Fe²⁺) and trivalent iron oxide (Fe³⁺) ispresent in the glass. Bivalent iron has a high absorptivity in thewavelength region at about 1 μm and up to visible wavelengths, whereastrivalent iron has a relatively low absorptivity in the visible shorterwavelength region of 400 to 500 nm.

When cerium oxide is included together with iron in the glass, then thephotochemical reaction Ce³⁺+Fe³⁺

Ce⁴⁺+Fe²⁺ is brought about by the UV light included in sunlight. Whenthis reaction occurs, then the transmittance both in the shorterwavelength region and the longer wavelength region is decreased.

As described above, near the bottom surface in particular, glass sheetsmanufactured by the float process contain trace components notoriginating from the glass raw materials, which have diffused from theoutside. The above-noted glass composition means strictly speaking thecomposition without those trace components. This composition can beknown from the rest excluding the surface portion near the surface ofthe glass sheet, for example the rest excluding the portion of up to 10μm depth from the surface of the glass sheet. Throughout thisspecification, the composition of this rest is referred to as bulkcomposition of the glass.

The bulk composition of the glass sheet of the present invention istypically a composition called soda-lime glass. Expressed in mass %,this composition includes for example, in addition to the above-notedextent of iron (preferably not more than 0.04 mass % in terms of Fe₂O₃):

65 to 80% SiO₂,

0 to 5% Al₂O₃,

0 to 10% MgO,

5 to 15% CaO,

10 to 18% Na₂O,

0 to 5% K₂O,

5 to 15% MgO+CaO,

10 to 20% Na₂O+K₂O,

0.05 to 0.3% SO₃, and

0 to 5% B₂O₃.

SiO₂ is the principal component forming the network of the glass. Ifthere is less than 65 mass % SiO₂, then the durability of the glass isdecreased, and if there is more than 80 mass %, then the glass becomesdifficult to melt.

Al₂O₃ is a component that improves the durability of the glass, but ifit exceeds 5 mass %, then the glass becomes difficult to melt.Preferably, it is in a range of 0.1 to 2.5 mass %.

MgO and CaO are used to improve the durability of the glass and toadjust the liquidus temperature and viscosity during forming. If the MgOexceeds 10 mass %, then the liquidus temperature increases. If the CaOis less than 5 mass % or more than 15 mass %, then the liquidustemperature increases. If the total of MgO and CaO is less than 5 mass%, then the durability of the glass is reduced, and if it exceeds 15mass %, then the liquidus temperature increases.

Na₂O and K₂O are used as melt accelerators. If there is less than 10mass % Na₂O or the total of Na₂O and K₂O is less than 10 mass %, thenthe effect of accelerating melting is poor, and if there is more than 18mass % Na₂O or the total of Na₂O and K₂O exceeds 20 mass %, then thedurability of the glass decreases. Since the raw materials of K₂O ismore expensive than those of Na₂O, it is preferable that it does notexceed 5 mass %.

SO₃ is a component that enhances the refining of the glass. If there isless than 0.05 mass %, then the refining effect with ordinary meltprocesses becomes insufficient and if there is more than 0.30 mass %,then the SO₂ generated by its decomposition tends to remain in the glassas bubbles or bubbles tend to be generated due to reboiling. Apreferable range is less than 0.15 mass %.

B₂O₃ is a component that is used to improve the durability of the glass,or also as a melting aid. If there is more than 5 mass % B₂O₃, thenproblems occur during forming due to the volatilization of the B₂O₃, sothat 5 mass % is taken as an upper limit.

The glass sheet of the present invention has the property that it issuitable as a component of a photoelectric conversion device. The glasssheet may be used as a cover glass sheet, or as a substrate on which thephotoelectric elements are formed. In the latter case, after previouslydepositing at least a conductive film serving as a front electrode, thephotoelectric elements and a metal film serving as a rear electrode maybe fabricated on the conductive film in this order.

As the conductive film, a thin film whose principal component is tinoxide, indium oxide or zinc oxide may be used, but a thin film whoseprincipal component is tin oxide to which impurities such as fluorine orantimony have been added is preferable. In this specification,“principal component” means a component that accounts for at least 50mass %. The sheet resistance of the conductive film is preferably 5 to15 Ω/square (Ω/□). In consideration of this, a preferable film thicknessof the conductive film is 500 to 2000 nm, and in particular 500 to 1500nm.

An undercoating film may be interposed between the substrate and theconductive film. An example of a preferable undercoating film is adouble-layer undercoating film, in which a first undercoating layer witha refractive index of 1.6 to 2.5 and a thickness of 5 nm to 100 nm, anda second undercoating layer with a refractive index of 1.4 to 2.0 and athickness of 5 nm to 100 nm are layered in that order onto thesubstrate. These undercoating films reduce the reflectance and thereflection interference colors, and moreover suppress the diffusion ofalkali components contained in the glass sheet into the conductive film.

It is preferable that the principal component of the first undercoatinglayer, which contacts the substrate, is at least one selected from tinoxide, titanium oxide, zinc oxide and aluminum oxide. It is preferablethat the principal component of the second undercoating layer is atleast one selected from silicon oxide, aluminum oxide, siliconoxynitride, silicon carbide and tin oxide. If the undercoating film istoo thin, then the effect of preventing the diffusion of alkalicomponents cannot be adequately displayed. On the other hand, if it istoo thick, then the effect of reducing the reflectance cannot beadequately displayed.

FIG. 1 is a cross-sectional view of an example of a glass sheet for aphotoelectric conversion device according to the present invention. Afirst undercoating layer 2 a, a second undercoating layer 2 b, and aconductive film 3 are deposited in this order on a glass sheet 1. Thethin films 2 a, 2 b and 3 may be deposited on the bottom surface, butare preferably deposited on the top surface of the glass sheet 1.

The thin films, such as the conductive film, may be deposited bysputtering or the like, but it is preferable that they are deposited bya process involving the thermal decomposition of a film-forming rawmaterial, such as CVD (chemical vapor deposition) or spraying. Here,spraying includes solution spraying, in which the raw material issupplied in liquid form, and powder spraying, in which it is supplied insolid form. In CVD, a film-forming raw material (for example tin rawmaterial) in the gaseous state and a reactive gas (oxidation rawmaterial) may be supplied via different pathways such that they reactnear the substrate, but it is also possible simultaneously to supply aseparating gas between the film-forming raw material in the gaseousstate and the reaction gas.

For the CVD process, it is possible to use a process in which thefilm-formation raw material is supplied to the surface of a glass sheetthat has been previously severed and heated to a high temperature (forexample 615° C. or higher), but it is possible to use so-called onlineCVD, in which a film is deposited on the glass ribbon in the tin floatbath during the glass manufacturing step in the float process, which ispreferable with regard to energy efficiency and film deposition over alarge area. Using this method, a film can be deposited on ahigh-temperature glass ribbon of about 620 to 750° C.

When online CVD is used, the conductive film is deposited on the topsurface of the glass sheet, so that the bottom surface of the glasssheet will be on the side of the photoelectric conversion device onwhich the light is incident. Consequently, in this photoelectricconversion device glass sheet, it becomes particularly important toreduce the reflectance at the bottom surface of the glass sheet. Thepresent invention presents a glass sheet for a photoelectric conversiondevice that includes, from one side, a glass sheet of the presentinvention and a conductive film deposited on the top surface of thisglass sheet.

FIG. 1 shows an example of an apparatus of online CVD. With thisapparatus, molten glass raw material is supplied from a melting tank(float furnace) 11 to a tin float bath (float bath) 12, and this rawmaterial is formed into a band-shaped glass ribbon 10 on the tin bath15. Moreover, a predetermined number of coaters 16 (in the embodimentshown in the drawing, there are three coaters 16 a, 16 b and 16 c) isarranged in the tin float bath 12, gaseous raw material is supplied fromthese coaters, and continuous thin films are deposited on the glassribbon 10.

Using a plurality of coaters, it is possible to continuously deposit aplurality of layers including a conductive film, for example anundercoating film and a conductive film, in that order on the glassribbon 10. After the thin films have been deposited, the glass ribbon 10is lifted up by a roller 17, and is fed into an annealing lehr 13. Theglass ribbon that has been annealed in the annealing lehr 13 is cut intoglass sheets of predetermined size by a cutting device that is not shownin the drawings.

It is also possible to deposit the conductive film using both online CVDand spraying. For example, it is conceivable to carry out online CVD andspraying in that order (for example by forming a film by CVD in the tinfloat bath, and forming a film by spraying at a stage downstream fromthat bath.)

Examples of the tin raw material that can be used in a thermaldecomposition process are stannous chloride and stannic chloride,wherein the latter is easier to handle and more stable than the former.It is also possible to use organic tin compounds such as dimethyltindichloride, monobutyltin trichloride as the tin raw material. Theseorganic tin compounds are preferable in that their reactivity is lowerthan that of stannic chloride. As the reactive gas (oxidation rawmaterial), water vapor, oxygen or a suitable combination thereof may beused. Water vapor is convenient in that it breaks down the tin chlorideraw material in a hydrolysis reaction. It is also possible to use air oran alcohol such as methyl alcohol or ethyl alcohol as the oxidation rawmaterial.

In order to improve the conductivity of the tin oxide film, antimony orfluorine may be added. Examples of antimony raw materials are antimonytrichloride and antimony pentachloride, and examples of fluorine rawmaterials are hydrogen fluoride, trifluoroacetic acid,bromotrifluoromethane, chlorodifluoromethane and the like. In order toincrease the conductivity, it is preferable to add fluorine. Preferablythe fluorine concentration in the conductive film is not greater than0.2 mass %. In this case, the refractive index of the fluorine-doped tinoxide film becomes about 1.9. It should be noted that this conductivefilm may also contain other trace components, such as silicon, aluminum,zinc, copper, indium, bismuth, gallium, boron, vanadium, manganese, orzirconium. However, it is preferable that the concentration of thesetrace components is not greater than 0.02 mass %.

If a thin film having silicon oxide as its principal component isdeposited by CVD, then it is possible to use monosilane, disilane,trisilane, monochlorosilane, dichlorosilane, 1,2-dimethylsilane,1,1,2-trimethyldisilane, 1,1,2,2,-tetramethylsilane,tetramethylorthosilicate, or tetraethylorthosilicate as the silicon rawmaterial. Suitable examples of the oxidation raw material for this caseinclude oxygen, water vapor, dry air, carbon dioxide, carbon monoxide,nitrogen dioxide, and ozone. In order to prevent a reaction of thesilane until it reaches the glass surface, it is possible to use inaddition an unsaturated hydrocarbon gas, such as ethylene, acetylene ortoluene.

If a thin film having aluminum oxide as its principal component isdepositeted by CVD, then it is possible to use trimethylaluminum,aluminum triisopropoxide, diethylaluminum chloride, aluminumacetylacetonate and aluminum chloride, as the aluminum raw material.Examples of suitable oxidation raw materials for this case includeoxygen, water vapor and dry air.

EXAMPLES

The following is a more detailed explanation of the present invention byway of examples, but the present invention is in no way limited to theseexamples.

A raw material was prepared such that the bulk composition of the glassbecomes as listed in Table 1. This raw material was melted in a meltingtank, poured into tin float baths holding various kinds of molten tinwith different iron contents and cleanliness, and thus formed to athickness of about 4 mm. These glass ribbons were cooled down in anannealing lehr, and cut further downstream. Thus, the glass sheets ofthe Examples 1 to 4 were obtained.

TABLE 1 component mass % SiO₂ 71.5 Al₂O₃ 1.5 MgO 4 CaO 8 Na₂O 14 K₂O 0.8SO₃ 0.2 iron oxide total 0.015 *That 100% is exceeded is due todiscrepancies in the number of significant digits.

In Example 1 to 4, the iron concentration in the molten tin, the degreeof cleanliness of the molten tin, the maximum values of the tin oxideconcentration and the iron oxide concentration near the bottom surface,the visible light reflectance at the bottom surface, the differencebetween the visible light reflectances at the bottom surface and the topsurface, and the transmittance at 540 nm and 450 nm before and afterirradiating UV light for 6 hours, as specified in the radiation-proofingtest according to JIS R3212 were measured. The results are listed inTable 2.

The iron concentration in the molten tin was measured by cooling andsolidifying samples of molten tin, dissolving it in hydrochloric acid,and performing a glow discharge emission spectral analysis on theresulting solution. For the cleanliness of the molten tin, a downstreamportion in the tin float bath was observed visually, and if tin oxidefloated on the molten tin, then it was determined to be “polluted” andif there was no floating tin oxide, then it was determined to be“clean.” In Example 1 and Example 2, the ratio of hydrogen gas tonitrogen gas supplied to the tin float bath was maintained at a levelhigher than in the prior art, to the extent that no tin oxide could beobserved.

The concentration of tin oxide and iron oxide near the bottom surfacewas measured by WDX analysis (acceleration voltage: 15 kV, samplecurrent: 2.5×10⁻⁷A, scan speed: 6 μm/min, analyzing crystal:Sn/PET,Fe/LiF) with an EPMA (JXA8600 by JEOL, Ltd.)

The visible light reflectance was measured by fabricating samples bygrinding the rear surfaces of the glass sheets such that they had a 5°inclination with respect to the surfaces of the front side, applying anon-reflective coating to the rear surfaces, and measuring these sampleswith a spectrophotometer (UV-3100 PC by Shimadzu Corp.) Also thetransmittance at predetermined wavelengths was measured with thisspectrophotometer.

TABLE 2 Ex. 1 Ex. 2 Ex. 3 Ex. 4 Concentration of iron in molten tin(ppm) 90 160 95 210 cleanliness of molten tin clean clean poll. poll.maximum value of tin oxide 0.71 0.88 2.35 3.40 concentr. (wt %) maximumvalue of iron 0.15 0.35 0.15 0.48 oxide concentr. (wt %) reflectance ofbottom surface (%) 4.1 4.2 4.4 4.6 reflectance difference betweenbottom/top 0.1 0.2 0.2 0.5 surface transm. at 540 nm (%) before UVirradiation 91.7 91.6 91.6 91.3 after UV irradiation 91.4 91.2 91.3 90.7transm. at 450 nm (%) before UV irradiation 91.4 91.3 91.3 91.0 after UVirradiation 91.0 90.6 90.9 90.2 *Ex. 4 is a comparative example

As shown in Table 2, the iron concentration and the cleanliness of themolten tin affect the optical properties of the glass sheets. In orderto reduce the reflectance of the bottom surface to below 4.15% and toreduce the difference to the reflectance at the top surface to 0.15%, itwas necessary to keep the iron concentration in the molten tin low andto maintain the molten tin clean, as in Example 1.

The bottom surface of the glass sheet in Example 1 was polished for atleast 10 μm from its surface, and a sample surface was fabricated bytaking away the layer including the trace components diffused thereintofrom the molten tin. However, the iron oxide concentration of thissample surface was still 0.15 mass %. This is, because there is a limit,due to noise, in the trace analysis of iron oxide with theabove-described quantitative measurement method. It is possible that inExample 1 and Example 3, the concentration of iron oxide near the bottomsurface is less than 0.15 mass % (that is, in Table 1, the 0.15 isactually “<0.15”).

As shown in Table 2, the correlation between the diffusion componentsnear the bottom surface and the properties of the glass sheet can beunderstood by an analysis using the EPMA. The quantitative analysis withan EPMA is a suitable means for analyzing whether the object of thepresent invention has been attained. However, in the analysis by EPMA,when quantifying a layer including a lot of tin oxide or iron oxide, thebeam diameter of the electron beam used in the analysis can be focusedto only a minimum of about 1 μm. This means, that the informationobtained with the EPMA corresponds to an average of about 1 μm diameter.To address this problem and to obtain concentration information at asmaller scale, the Sn concentration distribution from the outermostsurface to a depth of 1 μm of the glass sheet in Example 2 was analyzedby a secondary ion mass spectrometer (SIMS).

For the analysis by SIMS, a secondary ion mass spectrometer Phi-6600 byULVAC-PHI Inc. was used at the following primary ion beam settings: iontype: O₂ ⁺, voltage: 4 kV, current: 100 nA, angle of incidence: 60°,irradiated surface area: 200 μm², (detection region is 30% in the middleof the irradiated region), and sputtering rate: 26 nm/min. The SIMSmeasurement results were converted into Sn mass % in the followingmanner: A glass into which Sn ions have been implanted and that waspolished by 1 mm was used as a reference sample, and the Sn atom density(atoms/cc) in the sample was calculated from the calculated RSF(relative sensitivity factor), the ion intensity of the measuredanalytical elements and the intensity of the reference ions (principalcomponents). Then, the separately calculated atom density of the glassand the molecular weights were used to convert the atom density of Sninto mass %. As a result, it was confirmed that there is a concentrationmaximum of Sn between the outermost surface and a depth of 1 μm, andthat this maximum concentration is 1.5 mass % (1.9 mass % in terms ofSnO₂).

Thus, with the glass sheet according to the present invention, there isa concentration gradient of trace components near the bottom surface, sothat using analytical means other than EPMA, the maximum value of theconcentration of certain elements (compounds) may exceed the valuespecified in the claims. However, in any case, it is sufficient if, asin Table 2, the maximum value of trace components is evaluated by EPMA,as far as investigating the relation to the influence on the opticalproperties listed in Table 2 is concerned.

Example 5

Using the same device as in FIG. 2, an undercoating film and atransparent conductive film were deposited by online CVD in that orderon a glass ribbon. More specifically, the iron concentration in themolten tin was the same as in Example 1, and the ratio of the hydrogensupplied to the tin float bath was set so high that no tin oxide wasobserved in the molten tin.

From the first coater (16 a in FIG. 1) positioned furthest upstream, agas mixture of dimethyltin dichloride (vapor), oxygen, nitrogen andhelium was supplied, and a tin oxide thin film (refractive index: 1.9;first undercoating layer) of 55 nm thickness was deposited on the glassribbon. Subsequently, a gas mixture of monosilane, ethylene, oxygen andnitrogen was supplied from the second coater (16 b in FIG. 1), and asilicon oxide thin film (refractive index: 1.47; second undercoatinglayer) of 30 nm thickness was deposited on the first undercoating layer.Moreover, using the third coater (16 c in FIG. 2) arranged on thedownstream side, a gas mixture of stannic chloride (vapor), water vapor,nitrogen, helium and hydrogen fluoride was supplied at a glasstemperature of 630° C., and a transparent conductive film made of SnO₂:F (refractive index: 1.9; film of tin oxide doped with fluorine) of 750nm thickness was deposited.

For the resulting Example 5, as for Examples 1 to 4, the transmittanceat 540 nm and the transmittance 450 nm before and after 6 hours ofirradiation of UV light as specified in the radiation-proofing testaccording to JIS 212 were measured, and all transmittance changes werealmost the same as in Example 1. Moreover, the sheet resistance of theglass of Example 5 was 10 Ω/□, and the haze ratio was 12%. These valuesare preferable for glass sheets for photoelectric conversion devices.

As explained above, with the present invention, it is possible to obtaina glass sheet for photoelectric conversion devices that is preferablefor solar cells or the like, in which the reflectance of the glasssurface is kept low, and the inherent transmittance possessed by theglass can be sufficiently taken advantage of.

1. A glass sheet formed on molten tin, wherein a difference between avisible light reflectance of a surface formed in contact with the moltentin and a visible light reflectance of a surface on the opposite sidethereof is not greater than 0.25%; wherein, when the glass sheet'sthickness is 4 mm, a light transmittance at 540 nm wavelength is atleast 91.5%, and a light transmittance of at 450 nm wavelength is atleast 91.0%, and wherein after irradiating UV light for 6 hours asspecified in the radiation-proofing test according to JapaneseIndustrial Standard (JIS) R3212, a light transmittance at 540 nmwavelength is at least 91.0% and a light transmittance at 450 nmwavelength is at least 90.5%; wherein a maximum value of tin oxideconcentration in terms of SnO₂, in the glass sheet at a depth of up to10 μm from the surface formed in contact with the molten tin, measuredby a wavelength dispersive X-ray detector (WDX) mounted to an electronprobe micro-analyzer (EPMA), is not greater than 1 mass %; and wherein amaximum value of iron oxide concentration in terms of Fe₂O₃, in theglass sheet at a depth of up to 5 μm from the surface formed in contactwith the molten tin, measured by a wavelength dispersive X-ray detector(WDX) mounted to an electron probe micro-analyzer (EPMA), is not greaterthan 0.2 mass %.
 2. The glass sheet according to claim 1, wherein saiddifference of the visible light reflectance is not greater than 0.15%.3. The glass sheet according to claim 1, wherein both the visible lightreflectance of a surface formed in contact with the molten tin and thevisible light reflectance of a surface on the opposite side thereof isnot greater than 4.5%.
 4. The glass sheet according to claim 3, whereinboth the visible light reflectance of a surface formed in contact withthe molten tin and the visible light reflectance of a surface on theopposite side thereof is not greater than 4.25%.
 5. The glass sheetaccording to claim 1, wherein, in a rest without a portion up to a depthof 10 pm from the surface formed in contact with the molten tin, theglass sheet has a composition with an amount of iron oxide total interms of Fe₂O₃ that is not greater than 0.04 mass %.
 6. The glass sheetaccording to claim 5, wherein the composition is substantially free fromcerium oxide.
 7. The glass sheet according to claim 5, wherein thecomposition includes, in mass %: 65 to 80% SiO₂, 0 to 5% Al₂O₃, 0 to 10%MgO, 5 to 15% CaO, 10 to 18% Na₂O, 0 to 5% K₂O, 5 to 15% MgO+CaO, 10 to20% Na₂O+K₂O, 0.05 to 0.3% SO₃, and 0 to 5% B₂O₃.
 8. A glass sheet for aphotoelectric conversion device comprising the glass sheet according toclaim 1 and a conductive film deposited on that glass sheet.
 9. Theglass sheet for a photoelectric conversion device according to claim 8,wherein the conductive film is deposited on the surface opposite fromthe surface formed in contact with the molten tin.
 10. The glass sheetfor a photoelectric conversion device according to claim 8, furthercomprising an undercoating film disposed between the glass sheet and theconductive film.
 11. The glass sheet according to claim 1, wherein theglass sheet is formed on the molten tin in a float bath in an atmospheresuch that formation of the tin oxide in the molten tin is suppressed bymaintaining a ratio of hydrogen gas to nitrogen gas in the atmosphere ofthe float bath at a level in which no tin oxide is found in the moltentin.
 12. The glass sheet according to claim 1, wherein the ironconcentration in the molten tin is less than 100 ppm.
 13. The glasssheet according to claim 12, wherein the iron concentration in themolten tin is less than 95 ppm.
 14. The glass sheet according to claim12, wherein the iron concentration in the molten tin is less than 90ppm.